Various transportation systems, such as signaling systems, or systems that operate highway crossings, may detect the presence or approach of a vehicle (e.g., rail vehicle) to operate signals and/or crossings appropriately, and/or to schedule vehicle travel through a signal and/or crossing. Rail vehicles may be detected using a principle that axles of a rail vehicle that are electrically conductive will effectively short two rails of a track together. This shorting of two rails via an axle of a vehicle traveling over the rails is referred to herein as shunting. This shunting may de-energize track circuits for operating signals, or may be monitored for operating highway crossings. A track signal may be transmitted, for example, proximate to a signal, and that track signal may be received at a distance from the signal. A track signal may be transmitted, for example, proximate to a crossing, and a permanent shunt across the rails may be disposed at a distance from the crossing corresponding to an expected or permitted vehicle speed. The higher the speed, the farther away the permanent shunt may be. A track signal may be sent through a rail, with the signal traveling through the rail, through the track receiver or permanent shunt, and back toward the source of the signal. When a rail vehicle approaches the signal or crossing and crosses the permanent shunt so that the vehicle (or a portion thereof) is closer to the signal or crossing than the track receiver or permanent shunt is, the track signal is then routed back to the source via an axle (or axles) of the vehicle instead of via the permanent shunt. By monitoring, for example, the signal at the track receiver, the location of the vehicle between the transmitter and receiver may be detected, and a signaling system may be appropriately controlled. By monitoring, for example, a rate of change of electrical impedance of the circuit between the transmitter and the shunting, the approach of the vehicle within the distance to the permanent shunt may be detected, and a highway crossing system may be appropriately controlled.
However, the axles of a rail vehicle may not provide the desired shunting used to determine if the rail vehicle is within a predetermined range, for example, of a crossing or signal. For example, a running surface of the route (e.g., rails) and/or treads of wheels may be contaminated or covered by a non-conducting substance, such as a rust film. Shunting may be prevented until the non-conducting substance wears off, or wears off enough to allow shunting. If the rail vehicle fails to shunt, the presence of the rail vehicle may not be detected by crossing or signaling systems, resulting in delay, inconvenience, and/or reduced safety. Conventional approaches may use human inspectors who move along the track, for example, infrequently used lines, to inspect for rust or other material that may inhibit shunting. This manual inspection is slow and prone to errors.